1. Field of the Invention:
This invention relates to a method for preventing polymerization of an easily polymerizable substance in a thin-film type evaporating device.
2. Description of Related Art:
In the purification, recovery, etc. of an easily polymerizable substance, the polymerization of the easily polymerizable substance during such processes for manufacture of products of this substance by distillation, purification, and recovery constitutes itself a big problem.
In the production process of acrylic acid of high purity by the catalytic gas phase oxidation of propylene, for example, since the crude acrylic acid obtained during the process of separation contains such high boiling impurities as acrylic acid dimer and maleic acid, the practice of additionally incorporating a purifying process and a operating process for purifying the crude acrylic acid by separating and removing the high-boiling impurities has been prevailing. Since the high-boiling impurities so separated contain acrylic acid, the very product aimed at, the practice of recovering the acrylic acid for the sake of effective use has been in vogue. The easily polymerizable substance-containing solution which contains impurities, therefore, has been subjected to the operations of distillation and purification.
The contrivances devoted to the recovery of a product aimed at from the solution entraining high-boiling substances includes a thin-film type evaporating device. The method for purifying an easily polymerizable substance will be described below with reference to FIG. 8A and FIG. 8B which illustrate one mode of the conventional vertical thin-film type evaporating device. For a start, the device mentioned above is provided with a rotary shaft (4) which is disposed on a mechanical seal mounting plate (7) through a mechanical seal part (8) as illustrated in FIG. 8A. Generally, the rotary shaft (4) penetrates the interior of the devices and reaches a bearing part (15) in the lower part of the device. This rotary shaft (4) has a liquid distributor plate (5) connected thereto and is rotated in concert with the operation of the stirring rotary shaft (4). FIG. 8B represents a cross section taken through the line axe2x80x94a in FIG. 8A. A wiper (18) fixed with a supporting metal (20) and a rotor (17) for retaining a mist separator (26) are set in position through the distribution panel (5) as illustrated in FIG. 8B. When an easily polymerizable substance-containing solution is supplied through a raw material inlet (1), this solution is uniformly distributed to an evaporating heating surface (3) via a notched nozzle connecting the liquid distributor plate (5) by the centrifugal force originating in the operation of the rotary shaft (4). When a jacket (21) provided around the perimeter of the device is heated and the easily polymerizable substance on the evaporating heating surface (3) is consequently gasified, the low-boiling substance is gasified, transferred to the upper part of the device via a mist separator (26), and discharged through a vapor outlet (2). Meanwhile, the high-boiling substance is allowed to descend to the interior of the device and discharged through an extracted liquid outlet (10).
The feed material is moved from the liquid distributor plate (5) of the thin-film type evaporating device to the evaporating heating surface (3) and is spread out with a wiper (18) to form a very thin uniform liquid film. Since the liquid under treatment consequently has a short retention time and evaporates at a low temperature, the device is capable of evaporating a thermally unstable substance. Further, since the device is capable of forming a powerful thin film, it is applicable to a substance which produces a highly viscous residue after concentration and suitable for the distillation of an easily polymerizable substance.
As an approach to the purification of an alkyl aminoalkyl ester of acrylic acid or methacrylic acid, the official gazette of JP-B-06-53,711, for example, discloses a method for purification which comprises forming a thin film 0.2-2 mm in thickness and evaporating the ester through the thin film with an average retention time in the range of 2-40 seconds. The official gazette of JP-A-11-12,222 also discloses a method for recovering acrylic acid from an acrylic acid solution containing a high-boiling substance of high viscosity by the use of a recovering column which is provided with a thin-film type evaporating device.
Acrylic acid or methacrylic acid, however, is liable to polymerize at a high temperature. The polymerization not only decreases the quantity of purified acrylic acid or methacrylic acid to be obtained as a product but also entails serious problems such as solidification of a polymer in the evaporating device, for example. This polymerization tends to occur in the portion of the thin-film type evaporating device in which the viscosity is particularly high. Generally, the distillation of acrylic acid is carried out in vacuum of a high degree at the lowest permissible temperature such as not higher than 100xc2x0 C. for the purpose of preventing the polymerization, with a polymerization inhibitor such as phenothiazine or hydroquinone added to the distillation system. Since the boiling point of such a polymerization inhibitor is invariably higher than the boiling point of acrylic acid, consequently, the acrylic acid gas fails to include the polymerization inhibitor and the polymerization is not easily inhibited fully satisfactorily.
Further, the thin-film type evaporating device has a structural problem. Specifically, while the feed raw material is supplied to the evaporating heating surface (3) by the centrifugal force of the liquid distributor plate (5) annexed to the stirring rotary shaft (4) and subjected thereon to thermal distillation, the rotary shaft (4) is constantly kept in rotation during the operation of the device. This rotary shaft (4) penetrates the interior of the thin-film type evaporating device and connects with the rotary shaft driving part (9) disposed in the upper part of the device. The device is provided therein, for the sake of watertightness, with a mechanical seal (8) adapted for the rotary shaft. Though the mechanical seal (8) is one contrivance for preventing leakage of the vapor of distillation and, at the same time, keeping the motion of the rotary shaft (4) from obstruction, the vapor of distillation of an easily polymerizable substance is liable to adhere to the rotating part of the rotary shaft, forms a deposit of the polymer of the substance thereon, and consequently obstructs the rotation of the rotary shaft. The polymerization which is induced by the rotation of the rotary shaft (4) occurs also on the bearing part (15) in the device. Especially, at the extracted liquid outlet (10), the polymerization is liable to occur because the viscosity of the liquid in process is at a higher level than the feed raw material. Moreover, the extracted liquid outlet (10) tends to suffer adhesion thereto of the condensate liquid of high viscosity and entail stagnation of the extracted liquid in the lower part thereof. Once such adhesion of the polymer arises, the polymer obstructs the rotation of the stirring rotary shaft (4) and forms the cause for stopping the operation of distillation itself and the cause for decreasing the quantity of a product. As a result, the thin-film type evaporating device requires the interior thereof to be washed periodically and consequently puts an obstruction in the way of its lasting stable operation.
The thin-film type evaporating device can be used as connected to a distillation column or a recovering column. The use of this device in this manner may possibly arise in case of using the device as a re-boiler for reboiling the liquid extracted from a recovering column and, at the same time, supplying the vapor formed of a distillate component to the recovering column with the object of recovering an easily polymerizable substance aimed at through the top of the recovering column and, at the same time, circulating the liquid extracted through the bottom of the recovering column back to the thin-film type evaporating device. When the liquid extracted from the recovering column is used as the feed raw material to the thin-film type evaporating device, the wiper (18) will possibly seize on the evaporating heating surface (3) and induce the liquid under treatment to polymerize owing to an increase in the viscosity thereof. If this polymerization occurs, acquisition of a distillate component or condensate liquid having relevant components in fixed concentrations will become difficult and manufacture of a stable product will be obstructed.
The present inventors have discovered that polymerization arising in a thin-film type evaporating device can be effectively prevented by cyclically using a liquid extracted from the thin-film type evaporating device simultaneously with supplying a molecular oxygen-containing gas and adjusting the quantity of liquid per unit length of the evaporating heating surface of the device in the specific range. This invention has been established by the studies as a result. To be specific, this invention has the object of providing the following items (1) and (2).
(1) A method for preventing an easily polymerizable substance from being polymerized in a thin-film type evaporating device by supplying a solution of said easily polymerizable substance to the evaporating heating surface of said device by the centrifugal force of a stirring rotary shaft,
characterized by mixing the vapor of distilled vapor of the substance with a molecular oxygen-containing gas and supplying the extracted liquid obtained from the bottom of the device or a solution having lower viscosity than said extracted liquid to a feed raw material inlet thereby adjusting the quantity of a wetting liquid per unit length of the evaporating heating surface in the range of 0.02-2 m3/mHr.
(2) A method for effecting heat exchange on an easily polymerizable substance by supplying the distillate component at the vapor outlet to the recovering column disposed in the upper part of the thin-film type evaporating device and, at the same time, using at least part of the extract liquid of the recovering column as the feed raw material for the thin-film type evaporating device, characterized by disposing a liquid sealing part utilizing the extracted liquid of the recovering column in part of the pipe interconnecting the extract outlet of the recovering column and the thin-film type evaporating device or supplying the extracted liquid of the recovering column under pressure to the thin-film type evaporating device.